This invention relates to a power supply circuit and a power supply system for turning on/off a power supply by an external device or turning on/off a power supply together with a power supply of an external device.
FIGS. 1A and 1B show related power supply circuits. FIG. 1A shows a old circuit configuration. The power supply circuit has a power switch 112 arranged between a power plug 110 and a power transformer (main transformer) 113. In the power supply circuit, a power supply to the primary side of the power transformer 113 is turned on/off by manual operation of the power switch 112.
On the other hand, FIG. 1B shows a configuration described in patent document 1, etc. A power supply can be turned on/off by an external device of an infrared remote control, etc. A relay contact (NO contact) 123A of a power relay 123 is inserted between a power plug 120 and a main transformer 126. A subtransformer 124, a control section 125, and the relay 123 are included as a standby circuit for turning on/off a power supply by operating an external device. The subtransformer is connected directly to the power plug 120 and when the power plug 120 is inserted into a receptacle, the circuit is energized at all times. Therefore, the control section 125 connected to the secondary side of the subtransformer 124 operates at all times and is ready to receive a command from an external device of an infrared remote control, etc. Upon reception of a power on command from an external device, the control section 125 allows an electric current to flow into a relay coil 123B of the relay 123 for turning on the relay contact 123A. Accordingly, an electric current flows into the main transformer 126 and a load circuit connected to the secondary side of the main transformer 126 operates.
[Patent document 1] JP-A-2000-197268
However, a device incorporating the power supply circuit in FIG. 1A cannot be powered on/off by an external device and thus if the user wants to use the device at any desired time, the power switch 112 must remain turned on and power consumption is enormous. Since the control section 125 is turned on at all times by the subtransformer 124, the power supply circuit in FIG. 1B also involves a problem of consumption of standby power.